1. Field of the Invention
The present invention relates to a valve mechanism for an engine, which includes a switch assembly that switches a driving state of an intake valve or an exhaust valve of the engine.
2. Description of the Related Art
Conventionally, as a valve mechanism capable of switching the driving state of an intake valve or an exhaust valve of an engine, for example, there exists a valve mechanism described in Japanese Patent Laid-Open No. 2009-264199.
The valve mechanism disclosed in Japanese Patent Laid-Open No. 2009-264199 includes two types of rocker arms each of which changes the rotation of the cam of a camshaft into a reciprocal motion and transmits it to the intake valve or the exhaust valve, and a switch assembly that switches the driving state of the intake valve or the exhaust valve. The cam includes a first cam with a relatively large valve lift amount, and a second cam with a relatively small valve lift amount.
The two types of rocker arms are formed from a first rocker arm that is pressed by the first cam and swings, and a second rocker arm swingably provided at a position where the second cam can be pressed. The second rocker arm includes a pressing portion that presses the intake valve or the exhaust valve.
The switch assembly includes a slide pin that selectively connects the above-described two types of rocker arms, an actuator that applies an oil pressure to the slide pin, a return spring that returns the slide pin into one rocker arm, and the like. The switch assembly switches between a state in which the first rocker arm and the second rocker arm are connected to each other and integrally swing and a state in which the connection of the two rocker arms is canceled.
A pin hole through which the slide pin passes is provided in each of the rocker arms. The pin hole extends in the axial direction of the swing shaft of each rocker arm. The pin hole of the first rocker arm and the pin hole of the second rocker arm are formed at positions arranged on the same axis in a state in which the positions of the two rocker arms in the swing direction match.
The slide pin is pressed by the oil pressure and thus moves in the axial direction of the swing shaft of the rocker arm in the above-described pin hole against the spring force of a return spring. When the oil pressure disappears, the slide pin pressed and moved by the oil pressure is returned into one original rocker arm by the spring force of the return spring.
The first rocker arm and the second rocker arm are connected to each other when the slide pin moves to a connecting position across the rocker arms. The connected state is canceled when the slide pin is moved by the spring force of the return spring to a non-connecting position where the slide pin is housed in one original rocker arm.
When the slide pin is located at the connecting position, a driving force is transmitted from the first cam to the intake valve or the exhaust valve via the first rocker arm and the second rocker arm. On the other hand, when the slide pin is located at the non-connecting position, the driving force is not transmitted from the first rocker arm to the second rocker arm, and the driving force is transmitted from the second cam to the intake valve or the exhaust valve via the second rocker arm. For this reason, in the valve mechanism of the engine, the driving state of the intake valve or the exhaust valve is switched by changing the position of the slide pin.
In the valve mechanism described in Japanese Patent Laid-Open No. 2009-264199, to set the first rocker arm and the second rocker arm in the connected state, the oil pressure that presses the slide pin is applied to the slide pin. The time when the slide pin can move is the time when the first rocker arm and the second rocker arm have the same swing angle, and the pin holes of the two arms are arranged on the same axis. At a time when the pin holes are not arranged on the same axis, the slide pin cannot move, and therefore, the two arms are not connected. The time when the two arms have the same swing angle is the time when the intake valve or the exhaust valve is closed.
On the other hand, in a state in which the slide pin moves to the connecting position, and the driving force is transmitted from the first rocker arm to the second rocker arm, the slide pin is pressed against the hole wall surface of each pin hole by a force equivalent to the driving force. In this driving state, if a frictional force generated at the contact portion between the slide pin and the hole wall surface of the pin hole is large, the movement of the slide pin is regulated by the frictional force. Even if the oil pressure is canceled to return the slide pin to the non-connecting position by the spring force of the return spring in the driving state in which the large frictional force acts on the slide pin, the slide pin cannot move from the connecting position to the non-connecting position.
In the valve mechanism described in Japanese Patent Laid-Open No. 2009-264199, to cancel the connected state between the first rocker arm and the second rocker arm, first, the oil pressure applied to the slide pin located at the connecting position is canceled. In a case in which the driving force is transmitted from the first rocker arm to the second rocker arm, and the above-described frictional force is relatively large, the slide pin does not move even if the oil pressure is canceled. However, there is a time when the frictional force becomes small depending on a condition in which the two rocker arms swing. This time is, for example, the time when the intake valve or the exhaust valve lifts a little. In this case, since the reaction of the valve spring is small, the frictional force is small too. In addition, at the time when the intake valve or the exhaust valve is close to the maximum lift, the frictional force becomes small because a negative acceleration acts on the rocker arms. When the frictional force decreases, and the slide pin becomes movable by the spring force of the return spring, the slide pin moves from the connecting position to the non-connecting position.
In the driving device disclosed in Japanese Patent Laid-Open No. 2009-264199, a so-called “flip phenomenon” may occur in the process of canceling the connected state between the first rocker arm and the second rocker arm and in the process of shifting from the non-connected state to the connected state. The flip phenomenon is a phenomenon in which the connected state between the two rocker arms is canceled in a state in which the intake valve or the exhaust valve is not closed, and the second rocker arm and the intake valve or the exhaust valve are abruptly returned to the closing position by the spring force of the valve spring.
Two causes are considered to bring about the flip phenomenon, as will be described below. As the first cause, when the rocker arms shift from the non-connected state to the connected state, the rocker arms swing in a state in which the slide pin is insufficiently fitted. The slide pin is insufficiently fitted because the rocker arms are sometimes pressed by the cams and start swinging when the slide pin is slightly fitted in the rocker arms. If the rocker arms start swinging in the state in which the slide pin is insufficiently fitted, a load is applied to the slide pin fitting portion in a state in which the intake valve or the exhaust valve is open. When the fitting of the slide pin comes off due to the load, the flip phenomenon occurs.
As the second cause, probably, when the rocker arms shift from the connected state to the non-connected state, and the intake valve or the exhaust valve is open, the frictional force acting on the slide pin becomes small, and the fitting of the slide pin comes off due to the spring force of the return spring.
When the flip phenomenon occurs, an impact load is applied to the second rocker arm and the intake valve or the exhaust valve. If the flip phenomenon frequently occurs, the second rocker arm and the intake valve or the exhaust valve may be damaged.
For this reason, in the conventional valve mechanism in this type of engine, a transmission component such as the above-described slide pin is required to operate in a predetermined operation amount at a predetermined time and prevent the above-described flip phenomenon from occurring.